The invention relates to low-viscosity, low-monomer aqueous polymer dispersions based on polychloroprene and to a process for the preparation thereof and the use thereof as a contact adhesive.
The usual contact adhesives of the prior art based on polychloroprene, styrene/butadiene/styrene block copolymers or polyurethanes are predominantly solvent-containing adhesives which are applied to both substrates to be joined and are dried. After drying in air and subsequent joining of the two substrates under pressure, a joint structure having a high initial strength is obtained directly after the joining operation. If required, subsequent further physical and/or chemical crosslinking reactions increase the join strength. The adhesive formulations available on the market which are employed in this way have solids concentrations of organic components, comprising polymers, resins, plasticizers, anti-ageing agents and further conventional formulation auxiliaries, of from 10 to 25 wt. %, inorganic constituents, such as salts and fillers, of from 0 to 25 wt. % and solvent components of from 65 to 90 wt. %.
For ecological, economic and work safety and hygiene reasons, there is a growing demand for suitable aqueous dispersions which can be processed to corresponding solvent-free adhesive formulations.
Solvent-free contact adhesive formulations based on polychloroprene and, to a limited extent, natural rubber latex are prior art. Both are distinguished in that—analogously to solvent-containing contact adhesives—they are typically applied to both sides of the substrate. After a minimum drying or air-drying time specific for each of the formulations, during which excess water vaporizes/evaporates, the substrates are joined or pressed together, the adhesive film crystallizing under pressure or shear forces and a high initial strength being generated in this manner. Thereafter, analogously to the solvent-containing contact adhesives, further chemical and/or physical processes can be used to improve the joint strength. The usual variants here are, for example, post-crosslinking via blocked or latently reactive isocyanates and the use of suitable resins as formulation constituents.
A disadvantage of solvent-free aqueous contact adhesive formulations which is often mentioned, compared with solvent-containing contact adhesives, is the longer time span necessary to achieve, by evaporation/vaporization of the water, a concentration of the solids content of the joint component surfaces sufficient for film formation. A high initial solids concentration reduces the time until film formation of the adhesive dispersion on the joint components and therefore the waiting time until successful bonding by contacting of the joint components.
The lower initial strength, compared with solvent-containing systems, directly after the joining operation because of water enclosed in the adhesive film is mentioned as a further disadvantage.
To solve these problems, the prior art recommends adjusting aqueous adhesive formulations to a solids content which is as high as possible. The highest possible solids concentrations in the polychloroprene lattices employed are accordingly desirable for use as contact adhesives. For this reason, polychloroprene lattices which have proved suitable for adhesive uses are available on the market only with a solids concentration of greater than 40 wt. %. The possible solids concentration is limited purely physically by the densest particle packing of the lattices. The usual polychloroprene lattices have solids (polychloroprene) contents in the range of from 50 to 60 wt. %.
A desirable wet-in-wet bonding, which is defined as direct joining and bonding after application of the aqueous adhesive formulation, can be achieved by application of the adhesive by a 1- to 2-component process. In this context, lattices of the highest possible concentration are successfully employed (solids content >50 wt. %).
In a 1-component process, the adhesive formulation is adjusted by additional destabilization of the latex with electrolytes and/or lowering of the pH such that it breaks (forms a film) directly after application to the substrate and can be pressed or bonded.
For this purpose, in the prior art preferably anionic polychloroprene lattices stabilized via resin acids or disproportionated resin acids are destabilized with the aid of monovalent and/or divalent inorganic salts, such as, for example, KCl, NaCl, ZnCl2, MgCl2, ZnO, MgO and/or by inorganic and/or organic acids, such as, for example, HCl, boric acid, phosphoric acid, bicarbonate or acetic acid, glycine, other amino acids, tartaric acid, citric acid, or alkali metal and alkaline earth metal salts thereof, up to a point where they break directly under shear forces or pressure, form a film and can be bonded under pressure.
As described in US-A 2003/221778, increased initial strengths are achieved by the choice of rapidly crystallizing lattices and/or by the use of up to 30 parts of colloidally dissolved silica in combination with polychloroprene latex. Furthermore, diverse other formulation constituents, such as resins, polyurethane, acrylate and SBR lattices, organic solvents and additional oxidation stabilizers, fungicides, bactericides, ionic and nonionic surfactants, further fillers and the usual formulation constituents for aqueous dispersions can also be constituents of the formulation.
At solids concentrations of the adhesive formulations of greater than 45 wt. %, the formulations are reactive and high-viscosity formulations. Consequently, wet-in-wet bonding is faster, but with the disadvantage of a poorer storage/HCl and shear stability of the formulations.
WO 01/34718 discloses solids concentrations of below 35 wt. %. However, these polychloroprene lattices comprise acrylic acid esters or ethylvinyl acetate or colophony resin dispersions in combination with boric acid or amino acid or organic acid.
Formulation of adhesives which can be applied by the 2-component process is furthermore prior art. These adhesives are conventionally composed of an anionically stabilized polychloroprene latex and an activating component. By storing the activating component separately, on the one hand a high storage stability of the formulated polychloroprene latex component and on the other hand a high reactivity (rapid breaking of the adhesive formulation after application to the substrates) and as a result outstanding wet-in-wet adhesive properties after the components are combined are achieved. As an embodiment variant of this process, there is the possibility of application of the adhesive to the joint components by the “spray mix” process. In this process, the adhesive and a coagulating agent are conveyed separately into a spray gun, mixed in the spray jet and coagulated while still in flight and/or on the joint component. The viscosity of the latex component is adjusted here to viscosities of between 200-3,000 mPas, as is typical for application of contact adhesives by means of spray processes. In this context, concentrated aqueous solutions of alkali metal and/or alkaline earth metal ions and/or e.g. trivalent aluminium ions and/or inorganic/organic acids can serve as the activating component.
Disadvantages of this process are the high expenditure on apparatus, the mixing process, which is susceptible to error, and the typically high ion concentrations in the resulting adhesives, which lead to increased swelling with water of equilibrium or—in the case where acids are employed—to corrosion in connection with metallic substrates if low pH values are established. The process is comparatively time- and cost-intensive and is therefore unsatisfactory from the economic aspect.
All anionic commercially available polychloroprene dispersions stabilized with resin acids (or derivatives thereof, such as, for example, disproportionated resin acids or specific resin acid distillation cuts) split off significant amounts of HCl in the course of the storage time specified. In the case of ready-formulated, reactive 1-component adhesive formulations, this additional nuisance leads to an additional limitation of the storage stability, because the falling pH additionally destabilizes the formulation.
This problem and resulting possible discolorations of the aged adhesive formulations and/or of the adhesive applied and a possible acid attack by the HC liberated on any pH-sensitive substrates is conventionally solved by addition of divalent predispersed metal oxides (ZnO, MgO, CaO), in some cases present as nanoparticles, and/or aminic acid-trapping agents, such as hydroxylamine, ethanolamine or condensation products and derivatives thereof (WO A1 2004/106422). Anti-ageing agents against oxidative ageing are furthermore added.
The use of polyvalent metal oxides, such as MgO and CaO, is not desirable in anionic polychloroprene dispersions stabilized with resin acids, because the latex is destabilized. Concentrations higher than 0.24% of ZnO are subject to labelling in various countries and make the preparation of stable adhesive formulations having a pH of less than 10 considerably more difficult because of their amphoteric character.
Such formulations typically show only a short storage stability, especially in low-viscosity formulations, because of coagulation. In contrast, more highly viscous, for example paste-like formulations facilitate formulation of storage-stable systems, but greater changes in viscosity over the storage time are furthermore typical (increase by a factor of >2). Both variants, a stabilization via ions and also via the aminic stabilizers mentioned, are not advantageous for the swelling in water and the ecobalance. Thus, for example ZnO in concentrations of greater than 0.24 wt. % is classified as environmentally harmful, and all volatile organic constituents increase the TVOC (total volatile organic concentration) values of a formulation and therefore limit the use in applications with strict environment or fogging regulations.
In order to improve still further the adhesive properties and application properties (film formation) by means of a brush, roller or in the spray process, contact adhesive formulations are often adjusted to viscosities of 200-3,000 mPas for spray application and to 1,000-10,000 mPas for brush or roller application. This is conventionally effected by means of the usual thickeners, such as water-soluble polyurethanes, polyacrylic acids, polyols, cellulose/starch/sugar derivatives or polyacrylamides. In the case of colloidally dissolved or suspended silicas, this is effected by means of mono/divalent ions usually based on ZnO and by pH adjustment of the formulations to a value in the transition range of the colloidal stability of the aqueous silica solution/suspension (US-A 2003/221778). Since the viscosity of adhesive formulations thickened in such a manner is extremely pH-dependent, a long-term stability of the viscosity cannot be achieved in practice because of the progressive splitting off of HCl on the polychloroprene.
A serious disadvantage of the viscous and destabilized or activated formulations established in this way is in turn the often deficient storage, viscosity and shear stability, which easily leads to skin formation on the surface of the adhesive and to regular deposits, specks and lumps in the prepared adhesive formulations due to shear forces during transportation and application. Furthermore, the ecological problems during use of e.g. relatively high concentrations (up to 5 wt. %) of ZnO or boric acid, as described in EP-A 0 624 634 and Health Assessment no. 005/2006 of the BfR of 16 Nov. 2005, are to be noted.
The high solids content of approx. 50 wt. % in aqueous contact adhesive formulations compared with dilute solvent-based contact adhesives with a solids content of approx. 15 wt. % furthermore causes unnecessarily thick application layers of the adhesive. Under certain circumstances, this state of affairs adversely influences both the adhesive properties of the process and the economic characteristic data (material consumption) of the bonding.
Application of the adhesive formulation by means of spray processes is a preferred method for targeted thin application of contact adhesives to substrate surfaces, for example for foam-foam bonding in the furniture and mattress industry, and for uniform industrial wetting of surfaces for lamination in the automobile industry, as well as bonding of woven fabric and of leather. There are again and again problems here with so-called “overspray”. This means wetting with adhesive formulation on the other side of the target region and running off of excess spray composition from the substrate sample.
“Overspray” is a complex phenomenon which is partly caused by too low a viscosity of the formulation in relation to the application conditions—nozzle geometries and pressure ratios are to be mentioned here by way of example.
This low viscosity arises partly due to the discrepancy between the preparation viscosity, measured by means of a Brookfield rheometer at 12 or 30 or 60 revolutions/min at room temperature, and the viscosity under extreme shear stresses, such as occur in the usual spray units. The choice of suitable spray conditions for structurally viscous or thixotropic liquids is at best empirical in this manner without expensive measurement methods for the shear-related viscosity and in particular also the latex stability. Accordingly, the quality of the spray result is often deficient, and the amount of overspray is typically considerable. In this connection, repeated malfunctions due to blockages/bondings of the spray head because of the lack of shear stability of commercially available formulations also often occur (Technology for Waterborne Coatings ACS Symposium Series 663 1997 Chapter 15).
The use of silica products for various applications is known from the prior art. While solid SiO2 products are frequently employed for controlling rheological properties, as fillers or adsorbents, in the case of silica sols use as binders for diverse inorganic materials, as polishing agents for semiconductors or as flocculation partners in colloid chemistry reactions dominates. For example, EP-A 0 332 928 discloses the use of polychloroprene lattices in the presence of silica sols as an impregnating layer in the production of fireproofing elements.
US-A 2003/221778 furthermore describes the use of silica/water-glass suspensions for improving the wet tackiness and initial strength and final strength of polychloroprene contact adhesive formulations. Viscosities are moreover adjusted in a targeted manner via silica suspensions with the aid of the pH and mono- and/or divalent ions. The silicas disclosed in US-A 2003/221778 are sensitive to both low pH (<9) and high concentrations of divalent ions/metal oxides (ZnO, MgO). A stable formulation with respect to viscosity and/or latex stability based on polychloroprene latex containing colloidally dissolved silicas at a pH of <9.5 in combination with typical ZnO concentrations to adjust the viscosity and HCl or pH stability is known only in connection with additional auxiliary emulsifiers and/or stabilizing acrylate dispersions (company specification: Sales Aid—1-Component Spray Adhesives based on Dispercoll®C, Bayer MaterialScience June 2007).
According to the prior art, the use of these inorganic fillers in amounts of up to approx. 30 parts by wt., based on the content of solid in the polychloroprene latex, is advantageous. At higher concentrations, in addition to the initial and final strength in the 180° peel test, in combination with ZnO and at a pH of <10, the long-term stability of the formulations falls. At higher concentrations of silica sols, the possibility of stable adjustment of the viscosity by means of the usual thickeners or ZnO additions moreover disappears. An advantageous feature, as described, for use as a typical reinforcing filler and rheology auxiliary therefore already no longer exists at concentrations of from approx. 20 wt. %.
The term high initial strength is closely linked to the definition of a contact adhesive, Contact adhesives in principle can be applied at ambient temperature to substrates with the ambient temperature. A sufficiently high initial strength is to be understood as meaning a strength which is sufficiently high for further processing of joint components, so that slipping of the joined components during the subsequent processing process is avoided, or the bonded substrates having a strength below the adhesive and cohesive strength of the adhesive layer, so that as a rule when the joined object is subjected to load, the joint component is destroyed before the adhesive layer.
Pressure-sensitive adhesives are often also incorrectly called contact adhesives. However, these differ physically in that no immediately starting crystallization takes place in the adhesive layer in the joining process, which is why such bonds in principle creep under load. Typical examples of these are amorphous polyacrylate and polyurethane dispersions, formulations with a glass transition point of the amorphous polymer matrix below the application temperature. A disadvantage here is the creep process referred to, which does not allow permanent joining under load with the aid of such products and often does not achieve the required initial strengths.
Another example of incorrectly named contact adhesives are heat-activatable systems, e.g. so-called hot melts, which must be applied hot, joined immediately, and solidify on the cold joint component surface by crystallization or cooling to below the glass transition point. The necessary use of in some cases high-melting systems with a Tg/temperature of >>80° C. in order to ensure an adequate heat resistance is a disadvantage here. The use of high temperatures often presents problems on sensitive substrates. The user furthermore has to be protected from the high use temperatures. Application of the adhesive over a large area by means of spray systems presents problems because of the rapid solidification of the cohesion layer. Adhesive formulations on a hot melt basis moreover are typically hard and brittle after curing. A deficiency is furthermore derived in the use in comfort systems, such as furniture and mattresses. A sufficiently long open time which allows a flexible joining process also in complex working processes which take various times and a possible repositioning of the bonded elements likewise does not exist or is only inadequate.
Polyurethane-based adhesive dispersions which must first dry off on the substrate surface and form a film before they can be joined together after heat activation are moreover prior art. Latently reactive films/foils which can be laid on the substrate, activated by heat and then applied via pressing also function analogously. Disadvantages of the polyurethane-based products are the lack of wet-in-wet adhesive strength and the fact that application to vertical surfaces is ruled out since strength is achieved only after film formation and activation.